Cast aside - is not happy on the surface (6)
I believe the answer is:
disown
'cast aside' is the definition.
(I've seen this before)
'not happy on the surface' is the wordplay.
'not happy' becomes 'down' (down can mean sad).
'on' indicates putting letters inside (as in clothing 'on' a person).
'the surface' becomes 'is' (I can't explain this - if you can you should believe this answer much more).
'down' placed around 'is' is 'DISOWN'.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for disown that I've seen before include "Disclaim all connection or relationship with" , "Refuse to be associated with (something/someone)" , "Reject all connection with, refuse to acknowledge" , "Disinherit or renounce" , "Repudiate - disinherit" .)